Bunnies will sometimes randomly twitch in their sleep. I just presume it's from little bunny dreams. They will also sometimes do tooth chattering(not to be confused with tooth grinding from pain), and sometimes you can see them have hiccups, which can look a bit like they're twitching. These are all normal bunny behaviors.
The only other thing that could occur to look like twitching but is actually a health problem, is a seizure. But if you watch the videos in the link below, you can see that the twitching in a seizure is very different and can be much more intense.
A warning though: they are medical related videos that some may find disturbing.
http://wabbitwiki.com/wiki/Seizures